A turret punch press is designed to include an upper turret and a lower turret and to punch a plate form workpiece between a punch provided on the upper turret and a die provided on the lower turret.
On the other hand, it is possible to perform a side cutting for cutting a workpiece by continuously moving the above-mentioned punch up and down. As the side-cutting method, a method disclosed in Japanese Unexamined Patent Publication No., 63-130228 is known.
The side cutting method disclosed in the above-identified publication is that a punch provided with three stage edges, i.e. an upper edge, an intermediate edge and a lower edge, is driven up and down by means of a hydraulic cylinder for performing the side-cutting with the lower and intermediate edges, punching piece retained in a die is cut by means of the upper edge when a predetermined amount of the punching piece is retained, and the side cutting is again performed thereafter. The lower, intermediate and upper edges to be used for cutting are selected by controlling the stroke of the hydraulic cylinder.
However, in the above-mentioned side cutting method, since the punch is driven through a substantially small stroke to perform the side cutting with the lower and the intermediate edges, a dimension to be cut in a single stroke is relatively small and such as to require long process period. Furthermore, a saw-tooth like trace is frequently formed along the cutting surface to degrade the product quality.
Further, since the punching piece is cut while it is retained in a predetermined position in the die, a drooping can be created on the workpiece to degrade the product quality when the side cutting process is performed along a relatively long dimension.